1. Field of the Invention
The present invention relates to a navigation apparatus for use in a vehicle, and more particularly to an on-board vehicle navigation apparatus having a GPS (Global Positioning System) receiver that outputs coordinate data indicating the absolute position of the vehicle.
2. Description of Background Information
In recent years, navigation apparatuses which are for use in vehicles and which operate on the following principles have been developed and put into practical use. The typical navigation apparatus of this kind has a recording medium such as CD-ROM that contains map data including road data obtained by translating points on road segments into numerical form. While recognizing the current vehicle position, the apparatus reads from its recording medium a map data group of a region having a given area containing the current vehicle position. The data group is shown in a display device as the map indicating the current vehicle position and its surroundings. When displayed, the map automatically pinpoints the current vehicle position.
The above-described navigation apparatus has direction and distance sensors, the former acquiring the direction in which the vehicle is running and the latter obtaining the distance traveled by the vehicle. The direction and distance thus gained are used to estimate the current position of the vehicle in which the apparatus is mounted. Both the direction and distance data from the sensors generally contain a few percentage of error. That error is inevitably translated into a certain displacement of the current position of the vehicle estimated from the data relative to the actual position thereof.
Another kind of on-board navigation apparatus utilizes longitude and latitude information based on the data from a GPS receiver (called GPS data) to estimate the absolute vehicle position with an appreciably high degree of accuracy. The current vehicle position is then estimated according to the absolute position based on the GPS data. Because this kind of navigation apparatus has its GPS receiver receive signals from satellites at intervals of a few seconds, a scheme may be devised whereby the satellite-originated data are interpolated by the data from the direction and distance sensors mounted in the vehicle.
Some disadvantages are inherent in the above-outlined scheme. The fact that the vehicle, hence its GPS receiver, changes its position constantly makes it sometimes difficult for the receiver to keep an adequate positional relation with the available satellites or to receive signals from a sufficient number thereof. In such cases, the accuracy of the GPS data may be degraded. Inaccurate GPS data or low degrees of precision in road data may cause the vehicle position to be displayed off the road network on display, straining the credibility of the apparatus from the user's point of view.
The measuring of the position by the on-board vehicle navigation apparatus is usually accomplished by having its GPS receiver receive signals from three of four satellites. Where signals are received from three satellites, the two-dimensional position (referred to as two-dimensional position measurement hereinafter); where signals are received from four satellites, the three-dimensional position of the vehicle (in longitude, latitude and elevation) is measured (referred to a three-dimensional position measurement hereinafter).
One disadvantage of the above setup is that as the vehicle runs, the receiving position of the GPS receiver of the on-bard vehicle navigation apparatus keeps changing relative to the signal-emitting satellites. That means it is not always possible to continue receiving signals from four satellites for three-dimensional position measurement. Depending on its signal-receiving position, the GPS receiver may operate on signals from only three satellites, to start the two-dimensional measurement. In that case, the elevation value of the vehicle is not obtained. The inability to find where the vehicle is in terms of elevation can lower the accuracy of the current position coordinates, especially where the terrain has significant undulations.
One solution to the above-described problem is to describe beforehand the elevation values in map data. An obvious disadvantage of this scheme is that its implementation requires preparing impracticably large amounts of elevation data. Even if the data have been prepared, their huge quantities tend to either crowd out an appreciable amount of the map data from memory, or call for large memory capacities in which to store the road data temporarily.